Technical Field
The present disclosure relates to a sensor of volatile substances and to a process for manufacturing a sensor of volatile substances.
Description of the Related Art
Known to the art are humidity sensors of a capacitive type, which exploit as sensitive materials particular hygroscopic dielectric materials having an electrical permittivity that varies as a function of the degree of relative humidity. In practice, a sensitive layer of hygroscopic dielectric material is set between conductive structures coupled for forming the electrodes of a capacitor. The capacitance of the capacitor notoriously depends upon the electrical permittivity of the material that is set between the electrodes. Since this varies according to the humidity absorbed by the sensitive layer, the reading of the capacitance of the capacitor provides a measurement of the level of relative humidity in the environment.
Capacitive humidity sensors are much appreciated owing to their high sensitivity, good linearity over a wide range of values of relative humidity, low consumption, ease of miniaturization, and low manufacturing costs.
Some known types of capacitive humidity sensors use capacitors with plane and parallel plates. In this case, the electrodes of the capacitor are defined by parallel plates, and the sensitive layer is contained in a volume comprised between the electrodes. One of the electrodes of the capacitor, an external one, has through openings for enabling the environmental humidity to impregnate the sensitive layer. A limit of sensors of this type is represented by the rather slow response times. In fact, the exposed surface of the sensitive layer is small and is limited to the areas corresponding to the openings of the external electrode, which, on the other hand, cannot be increased beyond a certain limit without affecting the overall capacitance of the sensor.
Also sensors based upon comb-fingered capacitive structures have been proposed. In this case, two comb-shaped, coplanar and interdigitated electrodes are provided on a planar dielectric substrate and then coated with a sensitive layer of hygroscopic dielectric material, the electrical permittivity of which varies as a function of the humidity absorbed.
Sensors of this type have better response times as compared to sensors with plane and parallel plates, but the sensitivity is not very high because the portion of the sensitive layer capable of affecting the overall capacitance of the sensor is of small extent.
It would, instead, be desirable to have available capacitive sensors with a ready response, like sensors based upon comb-fingered electrodes, but with a higher sensitivity.